Equity Method Investments and Membership Interests in Joint Ventures (Schedule of Equity Method Investments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance	$ 104,676	$ 106,107
Sale of investment in affiliates	(1,879)
Advances to (distributions from) affiliates, net	(384)	(8,053)
Equity in comprehensive income (loss)	3,175	6,622
Ending Balance	105,588	104,676
Knight Hawk
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance	90,021	90,166
Sale of investment in affiliates	(1,879)
Advances to (distributions from) affiliates, net	(5,884)	(10,534)
Equity in comprehensive income (loss)	7,953	10,389
Ending Balance	90,211	90,021
DTA
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning Balance	14,655	15,941
Advances to (distributions from) affiliates, net	5,500	2,481
Equity in comprehensive income (loss)	(4,778)	(3,767)
Ending Balance	$ 15,377	$ 14,655